1. Field of the Invention
The present invention relates to a field of semiconductor-wafer manufacturing technology and, in particular, to a novel alkaline etching solution for silicon wafers, an alkaline etching method using the solution, a silicon-wafer manufacturing method using the solution, and further a silicon wafer obtained through use of the method.
2. Background Art
When a silicon wafer for use in integrated circuits, such as ICs and LSIs, and discrete semiconductor elements, such as transistors and diodes, the monocrystal obtained through a Czochralski method (CZ method) or a floating-zone method (FZ method) is cut off by using an inner-diameter-blade cutter or a wire saw, the edges are beveled, and afterwards a lap process (lapping) is performed for improving the flatness on a main surface with free abrasive grains, wet etching is performed for removing a process distortion applied to the wafer through the above-mentioned processes and then mirror polishing is performed. Such wet etching includes acid etching using a mixed acid of, for example, hydrofluoric acid, nitric acid, and acetic acid, and alkaline etching using alkali, such as sodium hydroxide or potassium hydroxide.
While acid etching has an advantage capable of controlling the etching rate and the state of the surface of the wafer after etching, a drawback of the large etching rate is a deterioration in flatness of the wafer which had been improved through the lapping process.
On the other hand, although the etching rate is slow, alkaline etching has the advantage of being capable of minimizing changes in flatness of the wafer obtained in the lapping process, thereby achieving a wafer with excellent flatness after etching. In recent years, to allow finer microfabrication, an extremely high-level flatness has been demanded for semiconductor wafers. For this reason, alkaline etching has been widely used.
In the alkaline etching technology that has been used so far, an alkaline component with a concentration smaller than 48 to 50 weight percent in an alkaline etching solution has widely been used. However, such widely used alkaline etching solutions exhibit anisotropy in which an etching speed on a (100) surface and an etching speed on a (111) surface are different from each other by approximately sixty to one hundred times. This poses a problem in which a concave portion (hereinafter referred to as a “facet”) occurs on the surface of the wafer, and asperities remain on the surface of the wafer after etching. If such asperities on the surface of the wafer are large (if the surface roughness is large), this poses problems with the occurrence of particles in the device manufacturing process and a decrease in productivity in a polishing process.
To address such problems, a technology disclosed in, for example, Japanese Patent Laid-Open Publication No. 2003-229392 has been suggested, in which a alkaline solution with a high concentration (equal to or larger than 50 weight percent) is used to control the etching speed and lessen the influence of anisotropy, thereby decreasing the size of the facet to improve the surface roughness. However, the alkaline solution with a high concentration may be frozen in winter, thereby posing a problem in view of convenience.
Thus, there has been strong demand for providing an alkaline etching solution for silicon wafers which is capable of improving the surface roughness of wafers without using the alkaline solution with high concentration.